Wide-angle lenses are commonly used in monitoring cameras and other image and video capture devices, where a large viewing angle is required. Although there is no universal definition of what constitutes a wide-angle lens, a viewing angle or Field of View (FOV) of approximately 100°-140° would be considered wide for the purpose of the present invention. Beyond about 100°, perspective distortions begin to appear in the image. Especially, there exist so-called Ultra Wide-Angle (UWA) lenses, with even larger Field of Views that can be up to or even greater than 180°. Image capture systems equipped with UWA lenses are used in many applications: surveillance and security monitoring cameras, video-conferencing systems, robotic vision systems, specialty photographic cameras, and vehicle camera systems, amongst others.
It is well known that images taken with UWA lenses exhibit significant amount of perspective distortions, also commonly known as fisheye distortions. When the image is displayed on a planar surface, straight lines would look skewed and curvilinear, more noticeably so for the objects farther away from the center of the lens. Moreover, the relative sizes of objects closer to the lens look exaggeratingly larger, so that different parts of an object appear out of proportion and perspective. It is also known that the perspective distortions are inherent in the mapping geometry, in a sense that they don't stem from any lens or system imperfection. These are a consequence of geometrically mapping a very wide FOV (>=180°) in the real world three dimensional (3D) space, to a two dimensional (2D) image space, that is an intrinsic feature of the lens.
How a UWA lens maps points in the 3D object space to the 2D image space will be referred to as the lens' mapping behavior or, in terms of a mathematical description, as the lens mapping. Identifying the lens mapping is a key step in calibrating a lens. The mapping is known a priori only in special cases, where the lens has a simple geometry or the mapping is provided by the manufacturer; otherwise the mapping must be determined. Further, to achieve a large angular FOV, manufactures may use a combination of several lenses or other optics. Yet further, mass produced UWA lenses most likely exhibit slight differences from lens to lens. It would be beneficial to be able to calibrate one lens against an ideal or standard behavior, or to calibrate lenses against one another.
Traditional calibration methods of using calibration test patterns comprising precisely positioned objects (lines, circles, rectangles, etc.) on a flat surface are generally not a viable approach for determining the mapping behavior of UWA lenses. A flat surface has limited field of view (<180°) that cannot cover full FOV of the lens.
Most existing prior art solutions for determining the lens mapping are models based on certain assumptions. For example, the ray optics of a real UWA lens can be traced and modeled in software. Thus, any deviation from the expected behavior can be calculated and corrected for. Another solution consists of direct measurement of a number of coordinates on a calibration target to compare against the observed image of the same coordinates and finding actual characteristic of the lens, such as focal length, and lens centroid. Such an approach requires time consuming measurements and expensive precision tools, e.g. an optical bench and/or a laser beam. Furthermore, this approach does not characterize the mapping behavior of the UWA lens, which may be needed in perspective correction applications.
The present approach utilizes mathematics of UWA lenses as an approximation of the expected behavior of the lens, and a custom design three dimensional test pattern in order to determine the precise lens mapping. Once the lens mapping is characterized, deviations from an ideal or standard behavior can be corrected using any known correction techniques. Also deviations between different lenses, that are supposed to be behaving the same, can be removed. In addition to lens calibration, the UWA lens maps can be supplied to systems that intend to correct for perspective distortions caused by the lens. For example, the co-pending patent application PCT/US2012/27189 discloses methods for perspective correction of UWA lenses, where the lens mapping transformation must be known.